1. Field of the Invention
The present invention relates to communication receivers. More specifically, the present invention is directed to multi-channel predictive decoding in a communication receiver.
2. Description of the Prior Art
Radio frequency (“RF”) homodyne receivers are used in RF identification (“RFID”) systems. RFID systems may be used, for example, to identify a vehicle passing through a toll lane to charge a toll fee to a corresponding account.
In addition to the receiver, an RFID system includes an RF source and a transponder tag associated with a target to be identified (e.g., a vehicle). The source transmits an RF signal that reaches the tag. The tag backscatter-modulates the signal, which is then received by the RF homodyne receiver. Backscatter modulation may be defined as the alteration of a tag's radar cross section with time.
The RF homodyne receiver mixes the backscattered signal with a local oscillator (“LO”) signal to produce intermediate frequency (“IF”) signals. The IF signals are then amplified and decoded to obtain identification data.
One of the major problems encountered with RFID systems is that to produce a measurable IF signal, the backscattered signal should preferably be in phase with the LO signal. Mathematically, if the LO and the RF signals received are cosine waves, the signals mix (e.g., are multiplied with each other) and can be expressed as a square cosine IF signal. But if the received RF signal is 90 degrees out of phase with the LO signal, the resulting signal is a cosine multiplied by a sine, which results in an IF signal having zero magnitude.
A first solution to the problem involves three-channel phase detection. In this approach, three different phases of the backscattered signal mix with the LO signal to produce IF signals. The receiver amplifies, filters, and digitizes the IF signals. The digitized IF signals are then sent to a decoder where information is extracted by selecting the two channels that have the same decoded data. The major problem with a two-channel decoder is trying to decide which of the two channels is correct. With the three-channel decoder only one channel can be out of phase at a time, and when two out of the three channels (phases) being received are in agreement, the correct data can be deciphered. The three-channel decoder however requires filtering and amplification of three independent channels, which takes up receiver space. In addition, the reception depends on the least sensitive of the two selected channels.
A second solution to the in-phase problem is similar to the three-channel phase detection approach discussed above. The difference lies in the use of a quasi-third channel. RF mixing occurs at four phase angles: 0 degrees (A+), 90 degrees (B+), 180 degrees (A−), and 270 degrees (B−). A channel “A” is processed as (A+)-(A−); a channel B is processed as (B+)-(B−); and the quasi-third channel is processed as (A+)-(B−). The quasi-third channel approach requires processing of three channels and two-out-of-three voting, which results in a significant decrease in the signal-to-noise ratio.
A third solution to the in-phase problem, which may be used with only two channels, is to correlate known symbols of choice with symbols received on each of two channels 90 degrees apart. The channel with the greatest correlation to a known symbol is then selected for decoding. Correlation works well when the exact periods of the incoming data symbols are known.
In RFID systems the tags are designed for low cost, and therefore, the period of the received symbols can vary by as much as 20%.
Not knowing the period (or length) of the incoming signal makes it very difficult to determine the period (or length) of the incoming signal utilizing correlation. This task becomes even more difficult when trying to choose between two channels.
A fourth approach for solving the in-phase problem is to distinguish a bad channel from a good one by selecting the signal with the greatest power. This is the technique normally utilized in sampled systems. For systems where amplitude information has been reduced to either ON or OFF, state power measurements are not available. This invention provides an advantage in systems where only binary transitions and their occurrence in time are available, including cases where the power levels of two incoming signals are not known.
The present invention as described in the sections that follow provides solutions to the problems discussed above.